Automotive driveline systems include complex gear trains and turbomachinery that rely on petroleum products to provide a hydraulic working fluid and lubricant. Specifically, passenger car automatic transmissions and transaxles use turbines, pumps, gears and clutches operating at high speed and high temperature in a lubricant. The high speed rotation and high power densities of these systems, combined with the air space in the system and air entrained in the lubricant, may result in the formation of foam. Foam, consisting of a small quantity of lubricant and a large quantity of air, compromises pump efficiencies by changing the compressibility of the lubricant. As a result, pistons and valves actuated by the lubricant may not function correctly if the air content of the working fluid is large. Furthermore, the gear trains may receive inadequate lubrication, due to low pump efficiencies and a reduced capacity for the lubricant to provide a cooling effect, if a foam condition exists. Modern designs of drivetrain hardware are trending towards small sumps and higher power throughput densities, and relying upon less lubricant in general than prior designs. A lower lubricant volume may compound the challenge of dispelling foam from drivetrain system under operating conditions over a period of time.
Most industries find it necessary to modify the lubricant with chemical additives to dispel foam, prevent air entrainment, or both. Such additives usually function by modifying the surface tension of the lubricant relative to air, and/or by introducing insoluble film-tension modifiers that promote the collapse of air bubbles in standing foam. The additives dispel air from the lubricant in a sump or similar location where air and lubricant interface by reducing the stability of the thin film of lubricant forming the walls of each bubble in standing foam.
A conventional method for evaluating transmission fluid additives includes the use of production vehicles, and a dynamometer and operation of the vehicles on the fluid containing the additives for many thousands of miles. The dynamometer test and vehicles may be used simulate extended and/or intense real-world service for the fluid and therefore may be used to evaluate additives for transmission fluids. However, in the case of antifoam additives, such conventional evaluation methods are extremely expensive and time consuming, particularly when evaluating a large number of different antifoam additives. Accordingly, there exists a need for a bench scale antifoam additive evaluation test providing results that correlate well with the expensive and time consuming vehicle tests.
According to embodiments of the disclosure, there is provided a method for evaluating antifoam additives for lubricants. The method includes a step of aging a quantity of fluid containing an antifoam additive in one or more high shear simulator(s) for a period of time to provide an aged fluid. After the aging step, the fluid is evaluated in a foam tendency evaluation test to provide the foaming tendency for the fluid.
In another embodiment, the disclosure provides a bench scale method for screening antifoam additives for lubricant compositions. The method includes aging a fluid containing an antifoam additive for at least about 5 hours in one or more high shear simulator(s), collecting fluid from the simulator(s) after aging foam testing the fluid, and determining a foaming tendency in the fluid.
In one embodiment, the high shear simulator(s) are selected from the group consisting of a tapered roller bearing simulator, a sonic irradiation tool, a fuel injector shear apparatus, and the like.
An advantage of the screening test according to the disclosure is that the test may provide a quicker and less expensive method for evaluating the ability of antifoam additives to dispel foam in a lubricant over the life of the lubricant. Another advantage of the screening test is that fluids containing antifoam additives may be evaluated under more closely controlled conditions than are possible with conventional vehicle tests thereby providing more accurate evaluation of the antifoam additives.